Display devices relying on use of liquid crystal compounds which are widely used therefor at present are driven by TN (twisted nematic) mode.
When this TN mode is adopted, however, there are involved such problems that in order to change the image being displayed, the driving time is prolonged, because the position of the molecule of the liquid crystal compound in the element must be changed, and also the voltage necessary for changing the position of the molecules of the liquid crystal compound, that is, the power consumption becomes large.
In distinction to switching elements utilizing TN mode or STN mode, the switching elements using ferroelectric liquid crystal compounds are able to function as switching elements only by changing the direction of molecular orientation of said liquid crystal compounds and hence the switching time required for operating the switching elements is markedly shortened. Further, because a value of Ps.times.E obtained from a spontaneous polarization (Ps) of the ferroelectric liquid crystal compound and intensity of the electric field (E) applied is an effective energy output for changing the direction of molecular orientation of said liquid crystal compound, power consumption required therefor can also be markedly minimized. Such ferroelectric liquid crystal compounds as mentioned above are suitable particularly as display devices for moving picture, because they have two steady states depending upon the direction of electric field applied, that is, bistability and also very favorable switching threshold value characteristics.
When these ferroelectric liquid crystal compounds are intended to use in optical switching elements, they are required to have such characteristics as an operating temperature in the vicinity of ordinary temperature or below, a wide operating temperature zone, a high switching speed and an appropriate switching threshold value voltage. Particularly, of these characteristics, the operating temperature range is especially important when the ferroelectric liquid crystal compounds are used in optical switching elements.
So far as ferroelectric liquid crystal compounds known hitherto are concerned, however, they are generally narrow in operating temperature, and even in ferroelectric liquid crystal compounds having a wide operating temperature range, said operating temperature range is in a high temperature zone excluding room temperature, as disclosed in R. B. Meyer et al., J. de Phys., Vol. 36 L, p. 69 (1975) and a paper reported by M. Taguchi and T. Harada, "Proceedings of Eleventh Conference on Liquid Crystal", p. 168 (1985). Thus, no ferroelectric liquid crystal compounds which are satisfactory from the standpoint of practical use are available yet.
Hopf discloses in U.S. Pat. No. 4,886,620 the compounds represented by the following formula I; EQU R.sup.1 --Q.sup.1 --A--(Q.sup.2).sub.q R.sup.2 I
wherein R.sup.1 may be an alkyl group of 1-15 carbon atoms (column 1, lines 11--13) or an alkoxy group of 3-12 carbon atoms (column 5, lines 4-8), Q.sup.1 and Q.sup.2 are independently represented by --(A.sup.0 --Z.sup.0)-- (column 1, lines 61-62) in which A.sup.0 may be hydroxynaphthalene (column 2, lines 16--17), Z.sup.0 may be --COO--, --OCO--or --CH.sub.2 CH.sub.2 --, A may be cyclohexylene or hydroxynaphthalene (column 1, lines 25--35), R.sup.2 may be --X--Q-- C*(Y)H--R in which X may be --COO--, --OCO-- or a single bond, Q is alkylene containing 1 to 5 carbon atoms, Y may be a methyl group, R is an alkyl group differing from Y and containing 1 to 18 carbon atoms wherein one or two nonadjacent CH.sub.2 groups may be replaced by such divalent group other than alkylene as --COO-- and the like.
As disclosed in this reference, the above type of the compounds indicating smectic phases has been considered to be required of having at least one of the groups of Z.sup.0, Z.sup.1 and Z.sup.2 being substituted by --CN group (See the reference at column 2, lines 24--44) in the prior art.
By the way, there have heretofore been made various proposals for light modulation elements using such ferroelectric liquid crystal compounds as mentioned above.
For example, these light modulation elements may be driven by a method using a liquid crystal cell composed of two transparent substrates being arranged so as to face each other, leaving a gap of about 2 .mu.m between said substrates, said gap being filled with a ferroelectric liquid crystal assuming a chiral smetic phase C.
The ferroelectric crystal has a layer structure in the chiral smetic phase C, and in this layer a major axis of molecule is oriented so that this axis forms a practically definite angle .theta. (called a tilt angle). In this state, as shown in FIG. 4, the major axis of liquid crystal molecule 41 gradually turns owing to interaction between the molecules to a different direction and comes to form a helical structure (FIG. 4).
However, when a gap of about 2 .mu.m formed by two glass substrates is filled with a liquid crystal material, the oriented state of the liquid crystal material is influenced by the glass substrates to release its helical structure, and the liquid crystal molecule 51 comes to exhibit two forms of steady state when viewed from above the transparent substrate 50 as shown in FIG. 5. In the steady state as mentioned above, because the major axis of liquid crystal molecule and a dipole perpendicular thereto take the direction opposite to each other in the two forms of steady state, the steady state of the liquid crystal material can be transferred between the above-mentioned two steady states by applying an electric field thereto.
In that case, the amount of transmitted light can be controlled by arranging the above-mentioned liquid crystal cell between two polarizing plates wherein the directions of polarized light cross at right angles so that the cell becomes dark (the amount of transmitted light decreases) when the liquid crystal in the cell takes one form of the two forms of steady state.
In the process as mentioned above, theoretically it is said that the steady state of liquid crystal material present in the cell involves only two forms as aforesaid. Therefore, it is said that when the liquid crystal material in the cell is once brought to the steady state by allocation thereto of an electric field, said liquid crystal material will not be transferred to another form of the steady state even when the electric field applied is eliminated therefrom, and accordingly the light modulation element comprising the above-mentioned liquid crystal cell comes to have a memory effect.
Actually, however, when the liquid crystal material held in a steady state is allowed to stand, as it is, without application thereto of an electric field, parts of the liquid crystal material are transferred sometimes to another form of steady state, and it is difficult to impart a sufficient memory effect to the light modulation element, that is, it is difficult to maintain the liquid crystal material in a definite steady state at its steady state for a long period of time with application thereto of an electric field. Therefore, in order to maintain the steady state of liquid crystal material, that is, a bright state and a dark state of the light modulation element, it is necessary to apply an electric field thereto to a certain degree.
In the conventional process as mentioned above, the application of an electric field is necessary for attaining even a dark state, and in most cases it was difficult to attain a dark state having a sufficient darkness. On that account, it has been unsuccessful in obtaining a sufficient brightness ratio of a bright state to a dark state, that is, a sufficient contrast.